Breeding Considerations for Rats

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Females rats should not be bred before 65 days of age.  They are

continuously polyestrous, which means that they come into heat

at fairly regular intervals (every 4-5 days) throughout the

entire year unless they are bred.  The period during which the

female is receptive to the male and allows breeding is about 12

hours and usually occurs at night.  Female rats can come back

into heat 48 hours after giving birth to a litter.  This is

called a postpartum estrus.  This period of receptivity is not

used when breeding rats because the breeding male is removed from

the enclosure just before the female delivers her litter because

of the high probability of injury to the new pups by the male.



After mating, a white, waxy substance, called a copulatory

plug, is visible within the female's vulva for 12-24 hours.  It

is not uncommon to find these plugs within the enclosure after

they have been discharged.



Pregnancy lasts an average of 3 weeks.  Litter sizes average 6-12

pups, though it is not unusual for a female's first litter to be

smaller in number.  Litter sizes decrease as breeding females

age.  Female rats should not be disturbed for the first few days

after delivery because stressed females may destroy their pups.

Excessive handling, loud noises, and even insufficient nesting

material have all been implicated with this destructive behavior.

Pups are usually weaned at about 3 weeks of age.  The female

resumes her breeding cycle 2-5 days after her pups have been

weaned.